TypestateLite for safe copy/destroy of RC Slice

[Iakh via Digitalmars-d]

Just another idea.

So as I understand 
https://github.com/dlang/DIPs/blob/master/DIPs/DIP1000.md#owning-containers
there is problem with safe assignment of slices. But the problem 
is actual only if some element has scope ref.

1. Lets introduce type state "referred".
```
//safe code
RCSlice!int a = ...; // a is not refferd
RCSlice!int c = ...;
{
scope ref e = a[0]; // a is reffered
...; // a is reffered
auto b = a; // b is a copy and not refferd
b=c; // in fact it is safe. assignment to b will not destroy "e"
}
...; // a is not refferd
```
2. function attribute @disable_if(arg)
this attribute will disable function (in the same way as @disable 
does)
depending on its arg.
opAssign(RefCountedSlice rhs) @disable_if(reffered) @trusted
This way assign can by trusted
For simplicity only allow @disable_if to depend on "this" state.
3.Forbid to create refs/ptr to var with typestate (only to pass 
as "this"). It would simplify typestate tracking. But it could be 
hard to define what is var with typestate.

So how do you think about all this?